1. Field
The present disclosure relates to a substrate for a surface light emitting device, the surface light emitting device, a lighting apparatus, a backlight including the lighting apparatus, and a method of manufacturing the substrate.
2. Description of the Related Art
Recently, flat panel type backlights have been actively developed. A typical surface light emitting device used for the flat panel type display devices is an organic electro-luminescence device (OLED). The OLED is a light-emitting device that uses electroluminescence of a solid phosphorescent material. However, the OLED has a stack structure in which materials having different refractive indices are stacked, and thus light irradiation efficiency of OLED to the outside by reflection through an interface (light extraction efficiency) is low.
When calculating a light extraction efficiency by simple calculation, a ratio between light which is confined in each layer and which thus cannot be extracted to the outside and light that is irradiated to the outside is about 45% for waveguide light that is confined in a transparent electrode or an organic thin film layer and cannot be extracted, and about 35% for substrate waveguide light that is confined in the substrate and cannot be extracted. Thus, just about 20% of emitted light may be extracted to the outside. The same result is disclosed in Advanced Material 6 (p. 491, 1994) (hereinafter referred to as “non-patent reference 1”).
Various studies have been performed to change the light emission angle by providing a means on a substrate of an OLED. Specifically, a diffraction grid structure may be provided on a substrate to increase light extraction efficiency by preventing reflection of light having a specific wavelength, or a lens structure may be provided on a surface of the substrate to obtain the similar effect. The structures above may provide an effect in increasing light extraction efficiency. However, these structures are substantially fine and complicated, and thus, the application of such fine and complicate structures in manufacturing process may not be practically and efficiently performed.
However, for example, in Japanese patent publication No. 2009-238507 (hereinafter “patent reference 1”), the light extraction efficiency is increased by reducing the thin-film guided-wave using a specific glass material having the same or similar refractive index as a transparent conductive layer. When a structure such as a lens is formed on a side of the substrate opposite to the organic thin layer through which light is emitted, the thin-film guided-wave still remains in the transparent electrode or the light emitting layer, and thus the thin-film guided-wave may not be extracted. However, in the method described in the patent reference 1, the thin-film guided-wave may be extracted. However, the substrate having a specifically high refractive index used in the patent reference 1 is substantially expensive for commercial mass production.
In another method of reducing the thin-film guided-wave, a structure that may convert a refractive angle using a diffraction grid or a diffusing structure is inserted between the substrate and the transparent conductive layer (e.g., an indium tin oxide (ITO)). In this case, since the manufacturing of a transparent electrode film along with the structure of the substrate may not be efficiently performed, a surface of a structure may be planarized using a material having the same refractive index as the transparent electrode.
For example, in Japanese Patent publication No. 1998-241856 (hereinafter “patent reference 2”), a method of manufacturing an inorganic light emitting device is proposed. In this method, a substrate having a non-uniform corrugate is used as a substrate of the inorganic light emitting device after smoothing a surface of the substrate using a spin on glass (“SOG”) material. Also, in Japanese patent publication No. 2003-297572 (hereinafter “patent reference 3”), a method of increasing light extraction efficiency of an OLED by reducing the thin-film guided-wave is proposed. In this method, an SiN film having a high refractive index with a thickness in a range from about 0.4 micrometer (μm) to about 2 micrometers (μm) is formed on a substrate having a surface roughness Ra in a range from about 0.01 μm to about 0.6 μm using a chemical vapor deposition (“CVD”), and the substrate on which the SiN film is formed is used as a substrate for manufacturing the OLED.
Also, in another method of reducing the thin film guided-wave light, for example, in International Publication No. WO 2009/017035 (hereinafter “patent reference 4”), a method of forming a glass layer that includes a diffusing component such as air and has a high refractive index between ITO and a substrate is proposed.
In addition, Japanese Patent publication No. 2010-198797 (hereinafter “patent reference 5”) discloses a method of manufacturing a glass substrate for an organic EL device in which a transparent conductive layer is formed on a surface of the glass substrate and an organic EL device is formed on the transparent conductive layer, wherein the EL device includes a glass substrate, on a surface of which a corrugated surface for diffusing light emitted from the EL device is formed, and a glass sintering layer that has a higher refractive index than that of the glass substrate and is arranged on the corrugated surface of the glass substrate, wherein the glass sintering layer planarizes corrugation of the corrugated surface of the glass substrate to provide a surface on which the transparent conductive layer is to be formed.